comp541 more on verilog; debouncing switches

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COMP541COMP541

More on Verilog;More on Verilog;Debouncing switchesDebouncing switches

Montek SinghMontek Singh

Feb 15, 2012Feb 15, 2012

First TopicFirst Topic More Verilog…More Verilog…

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HierarchyHierarchy AlwaysAlways make your make your

design modulardesign modular easier to read and debugeasier to read and debug easier to reuseeasier to reuse

before you write even one before you write even one line of Verilog…line of Verilog…

……draw a picturedraw a picture– black boxesblack boxes– boxes within boxesboxes within boxes

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module inv(input a,module inv(input a,

output y);output y);

assign y = ~a;assign y = ~a;

endmoduleendmodule

module and3(input a, b, module and3(input a, b, c, output y);c, output y);

assign y = a & b & c;assign y = a & b & c;

endmoduleendmodule

module nand3(input a, b, c output y); wire n1; // internal signal

and3 andgate(a, b, c, n1); // instance of and3 inv inverter(n1, y); // instance of inverterendmodule

Internal VariablesInternal Variables Internals = those that are not inputs/outputsInternals = those that are not inputs/outputs

declare them as declare them as wirewire or or regregdepending on whether they are combinational or state depending on whether they are combinational or state

holdingholding

p

g s

un1_cout cout

cout

s

cin

ba

module fulladder(input a, b, cin, output s, cout); wire p, g; // internal

assign p = a ^ b; assign g = a & b; assign s = p ^ cin; assign cout = g | (p & cin);endmodule

Bitwise Operators (we have used)Bitwise Operators (we have used)module gates(input [3:0] a, b,

output [3:0] y1, y2, y3, y4, y5);

assign y1 = a & b; // AND

assign y2 = a | b; // OR

assign y3 = a ^ b; // XOR

assign y4 = ~(a & b); // NAND

assign y5 = ~(a | b); // NOR

endmodule

CommentsComments

// single line comment

/*…*/ multiline comment

Reduction Operators (&)Reduction Operators (&)module and8(input [7:0] a, module and8(input [7:0] a,

output y);output y);

assign y = &a;assign y = &a;

// &a is much easier to write than// &a is much easier to write than

// assign y = a[7] & a[6] & a[5] & a[4] &// assign y = a[7] & a[6] & a[5] & a[4] &

// a[3] & a[2] & a[1] & a[0];// a[3] & a[2] & a[1] & a[0];

endmoduleendmodule

Reduction Operators (|, ~|, ~&, ^, ~^, Reduction Operators (|, ~|, ~&, ^, ~^, ^~)^~) Several others (see online reference)Several others (see online reference)

| = OR all the bits together| = OR all the bits together ~| = NOR all the bits together~| = NOR all the bits together ~& = NAND all the bits together~& = NAND all the bits together ^ = XOR all the bits together^ = XOR all the bits together ~^, ^~ = XNOR all the bits together~^, ^~ = XNOR all the bits together

Operator PrecedenceOperator Precedence

~ NOT

*, /, % mult, div, mod

+, - add,sub

<<, >> shift

<<<, >>> arithmetic shift

<, <=, >, >= comparison

==, != equal, not equal

&, ~& AND, NAND

^, ~^ XOR, XNOR

|, ~| OR, XOR

?: ternary operator

Highest

Lowest

NumbersNumbers Format: Format: N’BvalueN’Bvalue

N = number of bits, B = baseN = number of bits, B = base N’B is optional but recommended (default is decimal)N’B is optional but recommended (default is decimal) whenever in doubt, specify the # of bitswhenever in doubt, specify the # of bits

Number # Bits Base Decimal Equivalent

Value Stored

3’b101 3 binary 5 101

’b11 unsized binary 3 00…0011

8’b11 8 binary 3 00000011

8’b1010_1011 8 binary 171 10101011

3’d6 3 decimal 6 110

6’o42 6 octal 34 100010

8’hAB 8 hexadecimal 171 10101011

42 Unsized decimal 42 00…0101010

Bit Manipulations: splitting bits Bit Manipulations: splitting bits offoff

module mux2_8(input [7:0] d0, d1,module mux2_8(input [7:0] d0, d1,

input s,input s,

output [7:0] y);output [7:0] y);

mux2 lsbmux(d0[3:0], d1[3:0], s, y[3:0]);mux2 lsbmux(d0[3:0], d1[3:0], s, y[3:0]);

mux2 msbmux(d0[7:4], d1[7:4], s, y[7:4]);mux2 msbmux(d0[7:4], d1[7:4], s, y[7:4]);

endmoduleendmodule

mux2

lsbmux

mux2

msbmux

y[7:0][7:0]

s

d1[7:0] [7:0]

d0[7:0] [7:0]

s[3:0] d0[3:0][3:0] d1[3:0]

[3:0]y[3:0]

s[7:4] d0[3:0][7:4] d1[3:0]

[7:4]y[3:0]

Synthesis:

Verilog:

Bit Manipulations: packing bitsBit Manipulations: packing bitsassign y = {a[2:1], {3{b[0]}}, a[0], 6’b100_010};

// if y is a 12-bit signal, the above statement produces:

y = a[2] a[1] b[0] b[0] b[0] a[0] 1 0 0 0 1 0

// underscores (_) are used for formatting only to make it easier to read. Verilog ignores them.

Debouncing switchesDebouncing switches

What is switch bounce?What is switch bounce?

Let’s solve it in class!Let’s solve it in class!

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Switch bounceSwitch bounce Switches bounce or “chatter”Switches bounce or “chatter”

each press/release can bounce hundreds of times!each press/release can bounce hundreds of times!

Let’s design a chatter-free switch for Friday’s Let’s design a chatter-free switch for Friday’s lab!lab!

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Lab Preview: Buttons and Lab Preview: Buttons and DebouncingDebouncing Mechanical switches Mechanical switches

“bounce”“bounce” vibrations cause them to go to vibrations cause them to go to

1 and 0 a number of times1 and 0 a number of timescalled “chatter”called “chatter”

hundredshundreds of times! of times!

We want to do 2 things:We want to do 2 things: ““DebounceDebounce””: : Any ideas?Any ideas? Synchronize with clockSynchronize with clock

i.e., only need to look at it at the i.e., only need to look at it at the next +ve edge of clocknext +ve edge of clock

Ideas?Ideas?

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